In motor vehicles, systems are increasingly being integrated whose function is based on the rapid switching of inductive switching units with inductive loads. As a rule, these inductive loads are the induction coils of powerful, d.c.-operated solenoid valves. In order to switch off an inductive load quickly, it is necessary to transform the energy stored in its magnetic field as quickly as possible.
A commonly used circuit which accelerates the decay of the energy stored in an inductive load is a free-wheeling switch in series with a free-wheeling diode, the two together forming a free-wheeling circuit through which the inductive load can be short-circuited and which is connected in parallel to the inductive load and its main switch. In order to switch off the inductive load, the free-wheeling switch is closed at the same time as the main switch is opened. In so doing, the field strength of the magnetic field of the inductive load decays rapidly and the energy of the magnetic field is transformed into heat energy and as a result the switching off of the inductive load is completed at a faster rate. However, the energy stored in the inductive load is then completely lost; the power loss in the circuit arrangement is then maximum. Also, the free-wheeling circuit, in particular the free-wheeling switch, must be designed for the high voltage peaks that occur when the inductive load is short-circuited.
In circuit arrangements such as, for example, the so-called four-quadrant circuit according to U.S. Pat. No. 5,621,604 several inductive switch units, connected in parallel and supplied from a d.c. voltage source, can be switched independently of each other with a minimum of power loss. An inductive switch unit consists of an inductive load and a main switch between which a blocking diode, connected in the reverse direction to the minus pole, is contacted.
In the four-quadrant circuit, the power loss that occurs when switching off the inductive load is minimized by feeding back the current induced by the inductive loads in the circuit to the d.c. voltage source of the cicuit arrangement. Furthermore, the free-wheeling circuit, which is arranged with a free-wheeling diode in parallel to the inductive switch units, is connected through a free-wheeling switch to the minus pole of the d.c. voltage source.
When activating (i.e., allowing current to flow through) an inductive load, the main switch of the inductive switch unit concerned and the free-wheeling switch are closed. A current then flows from the d.c. voltage source through the main switch and the inductive load of the inductive switch unit concerned and through the free-wheeling switch to the minus pole of the d.c. voltage source.
After the free-wheeling switch has opened, the circuit arrangement is in the free-wheeling phase where the current flows in a closed circuit made up of the main switch and inductive load of the inductive switch unit concerned together with the free-wheeling diode.
To end the active circuit state of an inductive switch unit, the main switch of the inductive switch unit is opened. This causes the inductive load to induce a current in a circuit made up of the inductive load and the blocking diode of the inductive switch unit concerned together with the d.c. voltage source. As a consequence, the energy of the inductive load is fed back to the d.c. voltage source of the circuit arrangement with very little energy being transformed to heat and with a minimum loss of power in the circuit arrangement.
This circuit arrangement does, however, suffer from the serious disadvantage that the disconnecting time, within which the energy stored in the inductive load can be fed back to the d.c. voltage source, depends on the voltage of the d.c. voltage source of the circuit arrangement.
Consequently, the disconnecting time of the so-called four-quadrant circuit is increased drastically compared with the conventional circuit arrangement, especially when the d.c. voltage source supplies low voltages (below 20 volts).
The object of the invention is to provide an improved circuit arrangement that has advantageous properties with regard to the loading of the free-wheeling switch, the disconnecting time of the inductive load, and the power loss at the time of